Update Linux install instructions

[bkmgit]

Need to specify INSTALL_ROOT rather than PREFIX for custom install locations.

[what-the-diff]

PR Summary

• Simplified Installation Instructions
  We've made a major simplification to the installation instructions found in the .github/README-DEVELOPER.md file. Now, building Seamly's application will be easier and less complex! You won't have to use the longer qmake PREFIX=/usr/local Seamly2D.pro command that could get a bit intimidating. Instead, let's just focus on using make INSTALL_ROOT=/usr/local install, so we direct Seamly's installation binary files where they need to be. This change makes everything straightforward and user-friendly. Let's make tech less stressful and more fun! 😊🚀

[bkmgit]

Closes #1115

[slspencer]

@bkmgit You're recommending make instead of qmake. Qmake and make are not equivalent.
https://stackoverflow.com/questions/1368534/why-does-qt-use-its-own-make-tool-qmake
Have you compiled locally on a Linux distro and tested that the app is working (signals & slots are working), and that the executable is in usr/local/bin?